The present invention relates to process control apparatus. More particularly, it relates to improved digital input circuitry for use with a computer based process control system.
In the art of industrial process control, there have been provided systems wherein various parameters of the process are measured, compared with a desired value, and the difference then being used to derive a control or output signal. In a number of cases, both the input, or measured variable signals, and the output signals are in the form of analog signals. In some cases, however, the primary sensing element produces signals, representative of measured variables, in the form of digital signals. In such systems, a great number of input circuits share the single process control computer on a time scanning basis. In such systems, if all of the digital data is transmitted to the process computer for each of the digital input devices each time the devices are scanned, there would be a great amount of traffic on a communication bus system and tend to slow the response of the system to control the several processes.